Isolation structures are used in a variety of semiconductor devices to electrically isolate devices formed on a semiconductor substrate. Examples of isolation structures include both shallow trench isolation (STI) and deep trench isolation (DTI). In general, shallow trench isolation utilizes relatively shallow trenches filled with a dielectric material. Shallow trench isolation is commonly used to reduce parasitic capacitances and provide a relatively low level of voltage isolation between integrated devices. Conversely, deep trench isolation utilizes relatively deep trenches. For example, the trenches for deep trench isolation can extend to a buried insulator layer (e.g., BOX layer) in a silicon-on-insulator (SOI) substrate. In deep trench isolation, the trenches may be filled with a dielectric material, such as silicon dioxide, and in some cases additional materials, such as polysilicon. Deep trench isolation can provide relatively high voltage isolation between devices compared to shallow trench isolation. As such, deep trench isolation can be used to provide galvanic isolation between integrated devices on a semiconductor substrate. As such, deep trench isolation can be used to provide isolation between integrated devices that operate at different voltages levels and prevent undesirable interactions between distinct devices.
One issue with deep trench isolation is that in order to remain highly effective deep trench isolation needs to be implemented with one or more design rules. For example, allowing a “cross shape” of deep trench isolation has been shown to potentially reduce the manufacturability of the structure and therefore can reduce the yield, or percentage of acceptable structures in a design.
FIG. 6 illustrates an example of such cross-shape structures. FIG. 6 is a top view of a portion of semiconductor substrate 600. Formed in the semiconductor substrate 600 is a portion of deep trench isolation structure 602. The layout of the deep trench isolation structure 602 is such that it includes four cross-shaped intersections (e.g., cross shape 604). Cross shapes can occur anywhere four deep isolation trenches meet, typically at right angles.
Several manufacturability issues are related to cross shapes in deep trench isolation. As one example of manufacturability issues related to cross shapes, such cross shapes in the deep trench isolation structure can result in voids being formed in the dielectric material of the deep trench. In some cases, such voids can allow contamination to accumulate in the deep trench isolation structure, which can be transferred to subsequent manufacturing steps and degrade the effectiveness of the deep trench isolation structure.
There thus is a continuing need to improve the reliability of isolation structures, particularly deep trench isolation used in high voltage applications, and to have efficient layout techniques that accommodate the rule restrictions without using excessive additional area.